Diesel engine exhaust is a heterogeneous mixture which contains not only gaseous emissions such as carbon monoxide (“CO”), unburned or partially burned hydrocarbons or oxygenates thereof (“HC”) and nitrogen oxides (“NOx”), but also condensed phase materials (liquids and solids) which constitute the so-called particulates or particulate matter. Often, catalyst compositions and substrates on which the compositions are disposed are provided in diesel engine exhaust systems to convert certain or all of these exhaust components to innocuous components. For example, diesel exhaust systems can contain one or more of a diesel oxidation catalyst, a soot filter and a catalyst for the abatement of NOx.
A proven NOx abatement technology applied to stationary sources with lean exhaust conditions is ammonia Selective Catalytic Reduction (SCR). In this process, NOx (=NO+NO2) is reacted with ammonia to form dinitrogen (N2) over a catalyst typically composed of base metals. This technology is capable of NOx reduction greater than 90%, and thus it represents one of the best approaches for achieving aggressive NOx abatement goals. SCR provides efficient conversions of NOx as long as the exhaust temperature is within the active temperature range of the catalyst.
Reduction of NOx species to N2 using NH3 is of interest for meeting NOx emission targets in lean burn engines. A consequence of using NH3 as a reductant is that under conditions of incomplete conversion or exhaust temperature upswings, NH3 can slip from the exhaust of the vehicle. To avoid slippage of NH3, a sub-stoichiometric quantity of NH3 can be injected into the exhaust stream, but there will be decreased NOx conversion. Alternatively, the NH3 can be overdosed into the system to increase NOx conversion rate, but the exhaust then needs to be further treated to remove excess or slipped NH3. Even at a substoichiometric dosage of NH3, an increase in exhaust temperature may release ammonia stored on the NOx abatement catalyst, giving an NH3 slip. Conventional precious-metal based oxidation catalysts such as platinum supported on alumina can be very efficient at NH3 removal, but they produce considerable N2O and NOx as undesired side products instead of the desired N2 product. Thus, there is a need for a catalyst composition that is active for NH3 oxidation at temperatures as low as 225° C. and that has N2 selectivity in excess of about 60% between 250° C. and 400° C.
There is also a need for ammonia oxidation catalysts that are stable against the long term thermal, chemical, and physical stress of normal vehicle operation, which includes temperatures up to about 450° C. for a typical diesel application. In addition, a vehicle exhaust system may operate for short periods at temperatures above 800° C., for example during the thermal regeneration of a particulate filter. It is important that an ammonia oxidation catalyst be stable to these acute thermal stressors as well. For this reason, accelerated aging conditions are identified that mimics the cumulative effects of these long-term and acute stressors on the catalyst activity. Such an aging condition involves exposure of the catalyst to temperatures of 700° C. to 800° C. for between 5 and 50 hrs in the presence of up to about 10% water vapor in air.